C. Samson¹, S. Butler², C. Fry¹, P. J. A. McCausland³, R. K. Herd^1,4, O. Sharomi⁵, R. J. Spiteri⁵ and M. Ralchenko¹

¹Department of Earth Sciences, Carleton University, Ottawa, Ontario, Canada
²Department of Geological Sciences, University of Saskatchewan, Saskatoon, Saskatchewan, Canada
³Department of Earth Sciences, Western University, London, Ontario, Canada
⁴Earth Sciences Sector, Natural Resources Canada, Ottawa, Ontario, Canada
⁵Department of Computer Science, University of Saskatchewan, Saskatoon, Saskatchewan, Canada

Ten splash-form tektites from the Australasian strewn field, with masses ranging from 21.20 to 175.00 g and exhibiting a variety of shapes (teardrop, ellipsoid, dumbbell, disk), have been imaged using a high-resolution laser digitizer. Despite challenges due to the samples’ rounded shapes and pitted surfaces, the images were combined to create 3-D tektite models, which captured surface features with a high fidelity (≈30 voxel mm⁻²) and from which volume could be measured noninvasively. The laser-derived density for the tektites averaged 2.41 ± 0.11 g cm⁻³. Corresponding densities obtained via the Archimedean bead method averaged 2.36 ± 0.05 g cm⁻³. In addition to their curational value, the 3-D models can be used to calculate the tektites’ moments of inertia and rotation periods while in flight, as a probe of their formation environment. Typical tektite rotation periods are estimated to be on the order of 1 s. Numerical simulations of air flow around the models at Reynolds numbers ranging from 1 to 10⁶ suggest that the relative velocity of the tektites with respect to the air must have been <10 m s⁻¹ during viscous deformation. This low relative velocity is consistent with tektite material being carried along by expanding gases in the early time following the impact.

Reference
Samson C, Butler S, Fry C, McCausland PJA, Herd RK, Sharomi O, Spiteri RJ and Ralchenko M (in press) 3-D laser images of splash-form tektites and their use in aerodynamic numerical simulations of tektite formation. Meteoritics & Planetary Science
[doi:10.1111/maps.12287]
Published by arrangement with John Wiley & Sons

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Krisztian Fintor¹, Changkun Park², Szabolcs Nagy¹, Elemér Pál-Molnár^1,3 and Alexander N. Krot²

¹Department of Mineralogy, Geochemistry and Petrology, University of Szeged, Szeged, Hungary
²Hawai‘i Institute of Geophysics and Planetology, School of Ocean, Earth Science and Technology, University of Hawai‘i at Mānoa, Honolulu, Hawai‘i, USA
³MTA-ELTE Volcanology Research Group, Budapest, Hungary

We report an occurrence of hexagonal CaAl₂Si₂O₈ (dmisteinbergite) in a compact type A calcium-aluminum-rich inclusion (CAI) from the CV3 (Vigarano-like) carbonaceous chondrite Northwest Africa 2086. Dmisteinbergite occurs as approximately 10 μm long and few micrometer-thick lath-shaped crystal aggregates in altered parts of the CAI, and is associated with secondary nepheline, sodalite, Ti-poor Al-diopside, grossular, and Fe-rich spinel. Spinel is the only primary CAI mineral that retained its original O-isotope composition (Δ¹⁷O ~ −24‰); Δ¹⁷O values of melilite, perovskite, and Al,Ti-diopside range from −3 to −11‰, suggesting postcrystallization isotope exchange. Dmisteinbergite, anorthite, Ti-poor Al-diopside, and ferroan olivine have ¹⁶O-poor compositions (Δ¹⁷O ~ −3‰). We infer that dmisteinbergite, together with the other secondary minerals, formed by replacement of melilite as a result of fluid-assisted thermal metamorphism experienced by the CV chondrite parent asteroid. Based on the textural appearance of dmisteinbergite in NWA 2086 and petrographic observations of altered CAIs from the Allende meteorite, we suggest that dmisteinbergite is a common secondary mineral in CAIs from the oxidized Allende-like CV3 chondrites that has been previously misidentified as a secondary anorthite.

Reference
Fintor K, Park C, Nagy S, Pál-Molnár E and Krot AN (in press) Hydrothermal origin of hexagonal CaAl2Si2O8 (dmisteinbergite) in a compact type A CAI from the Northwest Africa 2086 CV3 chondrite. Meteoritics & Planetary Science
[doi:10.1111/maps.12294]
Published by arrangement with John Wiley & Sons

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John Chambers

Department of Terrestrial Magnetism, Carnegie Institution for Science, 5241 Broad Branch Road, NW, Washington, DC 20015, USA.

The Sun’s rocky planets come in a range of sizes. Mercury weighs in at barely 5% of Earth’s mass, perhaps because it formed under harsh conditions close to the Sun. Venus is similar in size to our own planet. However, the modest mass of Mars is perplexing. Current theories for planet formation can explain the gross features of the solar system, such as the dichotomy between its rocky and gas-rich planets, but there is no consensus on why Mars is almost a tenth the mass of Earth and Venus. Now, simulations by Izidoro et al. (1) show that Mars’ small size may date back to a partial gap in the solar nebula, the cloud of gas surrounding the young Sun.

Reference
Chambers J (2014) Forming Terrestrial Planets. Science 344:479.
[doi:10.1126/science.1252257]
Reprinted with permission from AAAS

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